1. Field of the Invention
The invention relates generally to gas lasers and in particular relates to an improved adjustable mirror assembly for a laser.
2. Description of the Prior Art
Lasers have highly reflective mirrors located at either end of a lasing medium for stimulating light emissions and reflecting those emissions between them. Various schemes have been devised to permit alignment of these reflective mirrors relative to the axis of the laser plasma tube. Initially, the mirrors were mounted outside the evacuated plasma tube, which was usually terminated with Brewster windows, and placed in separate housings which were mechanically referenced or aligned to the laser plasma tube.
Gas lasers are generally constructed by two techniques; the first technique utilizes a plasma tube bottle in which windows (usually Brewster angle windows) are attached to the vacuum envelope and in which the regenerative cavity mirrors are supported externally to the bottle. It is a relatively uncomplicated matter to align and adjust the externally mounted mirrors. If the mirrors are properly aligned, the laser will function, and function well. If not, however, either the laser will not function at all or function only minimally.
It is desirable to have a mirror assembly which is an integral part of the laser plasma tube, not just an appendage as some prior art lasers, in order to provide a more compact package. Also, an integrated plasma tube and mirror combination provides a more stable arrangement.
In the second technique, generally referred to as the "internal mirror" method, laser mirrors are attached directly to cylindrical metal housings at either end of the plasma tube which, in turn, are sealed to the vacuum bottle. The mirrors at either end complete the vacuum envelope. Each mirror is usually deposited on a cylindrical glass substrate which is sealed in place with a glass frit or glass solder by placing the entire laser tube assembly in a 450.degree. C. oven.
More and more the laser industry is turning to the internal mirror technique even though such a technique poses certain problems. In order to hard seal the mirrors into place, they must be either pre-aligned to the bore of the resonant or laser cavity before sealing or must be aligned after sealing, i.e., while the laser is in operation. Pre and post sealing alignment has been a difficult proposition.
One of the leading devices for adjusting a laser internal mirror is the well known mirror mount fitting according to U.S. Pat. No. 3,826,998. The mirror mount fitting includes a metal member having a weakened wall corrugation formed circumferentially therearound with a window closing the outer end of the fitting. The inner end of the fitting is made integral with the end of the laser tube and the reflecting mirror is formed on the closing window itself. The corrugation or single bellows-type formation on the fitting may extend inwardly or outwardly of the tube axis and is substantially weaker and of substantially lesser thickness than the remainder of the fitting wall. The fitting may be deformed at the corrugation such that the yield point of the material is exceeded to a sufficient extent to adjust the mirror to a desired position relative to the tube axis. There are several limitations of such a device. A drawback of this prior art device is that the corrugation must be deformed beyond the elastic limit of the material so that the mirror will be set in place and not moved. Another undesirable feature is that the plastically deformed cylinder must be shielded from being bumped or jarred because the deformed area is weak and the cylinder may easily be deflected if bumped.
Another prior art device is a mirror holder assembly composed of two cylindrical members which are axially assembled together to form a gas tight seal at the joint. One end of the mirror holder assembly is attached to the plasma discharge tube while a mirror is mounted to the other end. Each of the cylindrical members has a flange which is perpendicular to the axis of the cylinders and the two flanges are separated by 0.10 inch from each other. Screws extend through one of the flanges and press against the other flange. To adjust the mirror, the leverage of the screws is used to tilt the endmost cylinder thereby adjusting the alignment of the mirror. Such a device, although providing highly accurate and precise mirror adjustments, is costly to machine.